With the widespread use of Liquid Chromatography-Mass Spectrometry (LC-MS) systems for analyzing complex mixture of compounds around the world, ionization sources working under atmospheric pressure such as Electrospray Ionization (ESI) and Atmospheric Pressure Chemical Ionization (APCI) sources have been playing very important roles in the fields of food safety, environment protection and homeland security. However, the time consuming processes of sample pretreatment before conducting any analysis in a mass spectrometer prevent the techniques from being implemented on site with high speed. This issue was addressed and partially solved with the emergence of some pioneering direct analysis methods such as Desorption Electrospray Ionization (DESI) (Science, Vol. 306, page 471 (2004)) and Direct Analysis in Real Time (DART) (Analytical Chemistry, Vol. 77, page 2297 (2005)).
The two techniques use either charged droplets formed from the electrospray process (DESI) or mixture of ions and metastable gas molecules from a discharge chamber to interact with the analytes on a solid surface and bring the formed ions into a mass spectrometer. In DART the ions and metastable species from the source probe is also able to ionize vapors from a volatile sample directly.
A large number of techniques with the capability of ionizing samples under the atmospheric pressure without sample preparation have appeared since then. Atmosphere Solid Analysis Probe (ASAP) (Analytical Chemistry, Vol. 77, page 7826 (2005)) and Desorption Atmospheric Pressure Chemical Ionization (DAPCI) (US publication No. 2007/0187589) are another two methods closely related to the present invention. In ASAP a gas stream from a commercial source probe was heated and directed towards a solid sample located near the exit of a gas tube and the entrance of a mass spectrometer. The desorbed analyte were then ionized by a corona discharge needle nearby and delivered into the mass spectrometer. In the DAPCI method a stream of high speed gas was ionized when it exits a capillary tube with a sharp needle protruding from within. The ionization process in this case is the result of interaction between the ions formed by the corona discharge and the neutral species on the surface.
The three methods (DART, ASAP, and DAPCI) discussed above all involve using a DC voltage to generate a corona discharge from a sharp needle for creating ions to interact with samples either in the gas phase or in the condensed phase. One limitation for these corona discharge based methods is that the plasma is only visible at the tip of the discharge needle and therefore the sampling area for the analyte is very uncertain. Other direct analysis methods based on plasma technologies were also developed since 2005 and they do not have similar problems.
For example, Plasma-assisted Desorption Ionization (PADI) (Analytical Chemistry, Vol. 79, page 6094 (2007)) and Flowing Afterglow—Atmospheric Pressure Glow Discharge (FA-APGD) (Analytical Chemistry, Vol. 80, page 2654 (2008)) are the two techniques utilizing glow discharge as the source for generating ions from vapor/solid surface directly. Both methods use He as the discharge gas and share similar discharge current (tens of milliamps). In PADI the glow discharge was generated by a RF voltage with amplitude of hundreds of voltages whereas in FA-APGD a DC voltage of 500 V was used. Unlike those corona discharge based sources described previously, the glow discharge based sources such as PADI and FA-APGD normally have luminous plasma which extends from the exit of the gas to the sample, which makes the alignment of the sampling area easy.
Another type of direct analysis methods involving using the plasma as the ionization probe was developed recently in both Xinrong Zhang (Dielectric Barrier Discharge Ionization (DBDI)) (Journal of American Society for Mass Spectrometry, Vol. 18, page 1859 (2007)) and Graham R. Cooks' (Low Temperature Plasma (LTP), Analytical Chemistry, Vol. 80, page 9097 (2008)) groups. Both techniques share very similar mechanism though the geometries are different. As the names indicated, the two methods use dielectric barrier discharge to generate ions from the ambient air for further ionization of analytes on a surface, and the plasma from the discharge has a temperature close to the ambient temperature. RF voltages with amplitude of several kVs were used in these cases. Again, the plasma generated by this mechanism is visible and could be used for alignment purpose.
However, almost all the techniques described above with luminous plasma require high amplitude RF voltages and this makes the modification difficult for the current commercial ion source based on APCI and ESI which all use DC voltage for ionization. The only exception is the FA-APGD method which uses a DC voltage to initiate the glow discharge. This method though would need a chamber filled with He gas which increases the complexity of the source modification and also the temperature of the plasma is very high (400˜700° C.) that makes the control of experimental conditions difficult.
Therefore, a plasma ionization source for direct analysis is desired with minimum modification to the commonly available ambient ionization source such as APCI, and better yet this source is desired to render the possibility of generating a visible and extending plasma in order to easily locating the sampling areas.